The present invention relates to a voice coil motor which is classified as a kind of a linear motor used for conveying a head of optical (or magneto-optical) disk drive or magnetic disk drive, and more particularly to an improved voice coil motor which causes magnetic flux density of an air gap to become uniform.
An optical head of an optical (or magneto-optical) disk drive or a magnetic head of a magnetic disk drive is installed to move in the direction of the disk's diameter to seek information recorded in the disk. To make the head trace a desired track fast and precisely, a motor having a quick response and constant control function is required.
FIG. 1 illustrates a conventional and typical voice coil motor for use in conveying a head. The voice coil motor comprises a yoke member 10 forming magnetic paths, a moving coil 20 wound movably around central portion 11 of yoke member 10, permanent magnets 30 and 30' being adhered on the respective upper and lower portions 12 and 12' of the yoke member 10 and applying a magnetic field to moving coil 20. Two permanent magnets 30 and 30' are disposed with like poles opposing each other. An air gap G having a predetermined space is formed between each of permanent magnets 30 and 30' and central portion 11 of yoke member 10, and at least part of moving coil 20 is placed in air gap G. When current flows through moving coil 20, electromagnetic thrust works perpendicularly to the directions of the current and the magnetic flux of permanent magnet 30 and 30' so as to move moving coil 20. Here, the thrust working to the moving coil 20 should be constant so as to easily control the position of the moving coil 20, and the magnetic flux density distributed in the air gap should be uniform so that the thrust works in constant strength. Thus, for better control characteristic of the voice coil motor, reluctance of the yoke member forming the magnetic path of the magnetic flux should be considered to cause the air-gap magnetic flux density to become uniform.
As shown in FIG. 1, the conventional voice coil motor has yoke member 10 made of a single material. However, since, among magnetic flux f.sub.10 and f.sub.20 flowing through the yoke member, the length of magnetic path where central flux f.sub.11 and f.sub.21 pass and the length of magnetic path where fringe flux f.sub.12 and f.sub.22 pass are different, the reluctance of each magnetic force line is non-uniform, which brings out partial difference in the density of the magnetic flux. The difference will be proven in the following equation. ##EQU1## where P is the permeance of the permanent magnet's working point;
Lm is the thickness of the permanent magnet; PA0 Am is the area of the permanent magnet; PA0 f is a leakage coefficient; PA0 Lg is the length of the air gap; PA0 Ag is the area of the air gap; and PA0 R is reluctance.
That is, permeance P determining the working point of the permanent magnet is inversely proportional to reluctance R, and reluctance R is proportional to the length of magnetic path. This is because the permeance value depends on the length. Accordingly, as shown in FIG. 2, the conventional voice coil motor described above has non-uniform air gap magnetic flux density that is lower in the central portion and higher in the periphery, resulting in a bad control function.